The present invention relates to fastener driving tools, and more particularly to pneumatically powered fastener drivers, also referred to as pneumatic nailers.
In conventional pneumatic nailers, such as those disclosed in U.S. Pat. No. 3,638,532 incorporated by reference, the nailer is pressed down upon a workpiece needing a fastener, and in so doing, a workpiece contact element (WCE) is retracted relative to the tool housing. User depression of a trigger activates a trigger valve, which directs pneumatic pressure within the tool such that a piston is driven down a cylinder to impact and drive a fastener into the workpiece. When the trigger and the WCE are activated, there is always a leak path through the trigger valve, which keeps the main valve open and the piston/driver blade assembly biased downward. The '532 patent discloses the use of a multi-valve housing located near the tool trigger where continued depression of the trigger results in a single cycle or an automatic repetitive cycling of the drive piston. In practice, this construction has proved too expensive to be commercially successful.
In the course of driving the piston down the cylinder, the pneumatic pressure is diverted to a return chamber within the tool housing. Once the fastener is driven, the user typically releases the trigger, and/or lifts the tool from the workpiece, which allows the WCE to return to its start position. When either of these conditions occurs, the trigger valve closes and pressure in the return chamber pushes the piston back to the start position. As the trigger is released, the main system pressure closes the main valve on top of the cylinder to prepare for the next cycle. As the piston moves upwardly, the air on top of the piston is vented through the exhaust port in the center of the main valve. However, if the user does not release the trigger and the WCE remains depressed, the piston will not return to the start position. Piston return is activated by either the release of the trigger or of the WCE.
With conventional pneumatic nailers, if the user drives fasteners at too rapid a rate, for example when building cabinets and fastening an elongate strip forming a seam, the driver blade can scratch the workpiece as the tool is dragged along the seam being fastened. Such damage occurs as the user drags the tool from one workpiece location to the next. This is because the piston has not returned to the start position. Thus, conventional pneumatic nailers suffer from the potential for the piston to fail to sufficiently rapidly return to the start position.